Light and peptides

Light alters a peptide only after something in it absorbs the light. The peptide bond itself absorbs only below about 220 nm, outside what room lighting emits. Above that, absorption comes from side chains: the aromatic rings of tryptophan (near 280 nm) and tyrosine (near 275 nm), and, more weakly, disulfide bonds between cysteines.

Many of the damaging reactions are indirect. An excited chromophore passes its energy to oxygen, forming singlet oxygen, which then oxidizes a neighbouring residue. The documented products include:

• Tryptophan to N-formylkynurenine and kynurenine, the latter yellow.
• Methionine to methionine sulfoxide.
• Histidine, whose imidazole ring is oxidized.
• Cystine, whose disulfide bond can cleave.

Sequence therefore determines which of these routes are available. Epithalon (Ala-Glu-Asp-Gly) has no aromatic, sulfur-containing or histidine residue and absorbs very little above 220 nm. Tesamorelin carries methionine and tyrosine, so it has both an absorbing residue and an oxidizable one. These are statements about structure, not measurements of any lot.

Why copper complexes are a special case

Copper(II) in GHK-Cu absorbs in the visible range, which is what gives the complex its blue colour. Copper also catalyzes oxidation, so the complex holds both a light-absorbing centre and a metal that can drive oxidative chemistry. Solara's handling guidance records that copper complexes are photosensitive; no photostability data for GHK-Cu are published.

How photostability is tested

The harmonized reference method is ICH Q1B. Confirmatory exposure is an overall illumination of not less than 1.2 million lux hours and an integrated near-ultraviolet energy of not less than 200 watt hours per square metre. A dark control held alongside separates change caused by heat from change caused by light. Exposed and control material are then compared for appearance, assay and degradation products, which for peptides usually means HPLC.

Solid versus solution, and shielding

A lyophilized cake is generally more resistant than a solution of the same peptide. Molecular mobility in the dry solid is low and there is no dissolved oxygen, and both slow photochemistry. Resistant is not immune: absorption still occurs, and residual oxygen in the vial headspace remains a variable.

Shielding works on the light, not the chemistry. Amber glass cuts transmission below roughly 450 nm, which covers the near-ultraviolet and blue range, and an opaque carton removes the remainder.

Solara's site guidance is to store the lyophilized powder at −20 °C, protected from light. Solara has not published photostability data for any lot, so its light-protection guidance is general handling practice, not a result measured on Solara's material.

What failure looks like